extensibility strawman

I've put up some strawman text for a spec on extensibility.  It's quite
terse, so it may not make a lot of sense.  It also has a macro feature
which might be overkill but which I think is worth considering.  (The
macro substitution feature would allow for many syntactic-sugar
extensions to be understood automatically.)

I put it here for now:
 
    http://www.w3.org/2005/rules/wg/wiki/Arch/Extensibility2

Comments welcome, of course.

   -- Sandro